The present invention relates to an apparatus for and a method of synchronising oscillators within a communication system. An example of such a system is a radio telemetry system where incoming data at a baseband frequency is frequency converted for transmission at radio frequencies.
In a radio based communication system, input data, for example speech or signals from a sensor generally occupy a relatively low frequency range, referred to in the art as the baseband. The baseband signals are frequency up converted for transmission over the radio link. Frequency up conversion can be performed by mixing (ie multiplying) the baseband signals with a frequency derived from a local oscillator.
Assuming, for the purposes of simplicity, that the baseband signal comprises a continuous tone having a frequency xcfx89B, and that the local oscillator has a frequency xcfx89L, then it is well known in the art that the result of the mixing will produce signals at frequencies xcfx89Lxe2x88x92xcfx89B and xcfx89L+xcfx89B this mixing process gives rise to the two side bands which are centred on the local oscillator frequency xcfx89L. One or both of these signals may then be transmitted in accordance with known transmission schemes. Assuming for simplicity that only the upper side band is transmitted, then the receiver is arranged to recover the baseband by mixing the incoming signal (xcfx89L+xcfx89B) with a signal xcfx89Lxe2x80x2 derived from a local oscillator.
This will produce signals at the sum and difference frequencies xcfx89Lxe2x80x2+xcfx89L+xcfx89B and xcfx89Lxe2x88x92xcfx89Lxe2x80x2+xcfx89B. If xcfx89L and xcfx89Lxe2x80x2 are identical then the recovered signal would be at the frequency xcfx89B (the term xcfx89Lxe2x80x2+xcfx89L+xcfx89B being ignored as it occurs outside a system passband). However if the local oscillators are not accurately matched then the term xcfx89Lxe2x88x92xcfx89Lxe2x80x2) will be non zero and a frequency error would be introduced into the recovered signal. The above discussion has ignored the contribution of the phase shift, but this is also significant and in order to accurately recover the baseband signal the outputs xcfx89L and xcfx89Lxe2x80x2 from each of the oscillators must be accurately matched both in frequency and phase.
A seismic data transmission system must recover amplitude, frequency and phase information from each of the geophones (or hydrophones) if the subsequent data processing for normal move out correction, dip move out correction and the like is to be accurately performed. It is important that the frequency of the local oscillators in the transmitters and receivers in such a radio telemetry system are very accurately matched. One approach to achieve this is to use a highly stable and accurate frequency reference in each receiver and transmitter unit. However, if frequency errors of approximately one part per million or less are required over a wide temperature range, then high accuracy temperature controlled references are required, these tend to be expensive, bulky and power hungry and are therefore not suitable for use in portable battery powered radio units.
U.S. Pat. No. 4,188,579 describes a system wherein a local oscillator in a receiver is tuned to a transmitter oscillator by referring to two pilot signals in the received signals.
According to one aspect, the invention provides data communication system comprising a master unit containing a master clock and at least one remote unit, the or each remote unit having a local oscillator, in which the master unit transmits a signal containing at least first and second pilot signals at predetermined frequencies, and in which the or each remote unit is arranged to receive the pilot signals, to compare the frequencies of the received pilot signals with a local record of their frequencies, and to adjust the frequency of the local oscillator to reduce the difference between the received and expected pilot signal frequencies to below a threshold, wherein the received pilot signals comprise a pilot tone recovered from both the upper and lower sidebands of a carrier signal comprising the received signals.
According to another aspect, the invention provides data communication system comprising a master unit containing a master clock and at least one remote unit, the or each remote unit having a local oscillator, in which the master unit transmits a signal containing at least first and second pilot signals at predetermined frequencies, and in which the or each remote unit is arranged to receive the pilot signals, to compare the frequencies of the received pilot signals with a local record of their frequencies, and to adjust the frequency of the local oscillator to reduce the difference between the received and expected pilot signal frequencies to below a threshold, wherein the pilot signals comprise two pilot tones and the pilot signal comparison comprises locating the received pilot signals by looking for two tones in the received signals which are separated by the frequency difference between the pilot tones.
According to another aspect, the invention provides a data communication system comprising a master unit containing a master clock and at least one remote unit, the or each remote unit having a local oscillator, in which the master unit transmits a signal containing at least first and second pilot signals at predetermined frequencies, and in which the or each remote unit is arranged to receive the pilot signals, to compare the frequencies of the received pilot signals with a local record of their frequencies, and to adjust the frequency of the local oscillator to reduce the difference between the received and expected pilot signal frequencies to below a threshold, wherein the pilot signal comparison comprises using a frequency shifted copy of the received signals to locate the received pilot signals in the received signals.
The invention also relates to corresponding methods of synchronising oscillators.
It is thus possible to provide a communication system in which the remote units can be provided with relatively inexpensive local oscillators which are controlled so as to lock on to a master oscillator.
Preferably the pilot signals have a narrow frequency spread and may be regarded as pilot tones.
Preferably the pilot tones are inserted into a baseband signal which is modulated by the transmitter. The pilot tones may both occur in the or each sideband of the transmitted signal. They may be modulated to include information for identification purposes or information transfer purposes.
Advantageously the power of the received pilot tones may be used to estimate the transmission loss occurring between the base station and a remote unit. This estimate may then be used to control the transmit power of the remote unit, in order to ensure that it""s signal is received with sufficient, but not excessive, power at the base station.
Preferably the or each remote unit is arranged at least intermittently, to monitor the signal from the master unit and to demodulate the signal therefrom to recover the baseband signals and hence the pilot tones. The or each remote unit down converts the incoming radio frequency signal by mixing it with a local oscillator. The or each remote unit includes a quadrature detector within its radio receiver such that both the in-phase and quadrature components of the baseband signal are output from the detector.
A signal processor is arranged to locate the pilots tones within the demodulated baseband signal and to use these to adjust the frequency of the local oscillator. Such signal processing may be performed in the analogue or digital domain and in hardware or software. Digital signal processors are now available at reasonable costs and are especially suited to this processing task. Preferably the signal processor is arranged to identify the pilot tones by looking for the known frequency difference between the pilot tones.
The use of two pilot tones for frequency locking of the local oscillator significantly enhances the reliability of the system. Each local unit is likely to be employed in a potentially noisy environment (in terms of electrical and radio frequency interference), and consequently a phase locked loop seeking to lock to a single pilot tone would run a significant risk of locking to a spurious or noise signal. By seeking to lock to two tones separated by a known frequency, this risk is significantly reduced. For example, in a transmission scheme transmitting both upper and lower side bands, the pilot tones may be equally spaced about the carrier frequency. Thus the pilot tones may be correlated with respect to one another. Whilst the concept underlying the invention can be more easily envisaged when the pilot tones have discrete positive frequencies in the baseband such that both tones occur in a, or each, sideband, it will be appreciated that frequency down conversion gives rise to the possibility of one or both of the pilot tones having a negative frequency. From consideration of a geometrical representation of cyclical motion, where an oscillating variable is represented as a line of fixed length undergoing circular motion and the in-phase and quadrature components are represented by the projections onto the orthogonal X and Y axes of the representation, then it will be observed that a positive frequency has the following components:
and a negative (ie counter rotating) frequency has the following components:
Thus, in a system where phase information exists, negative frequencies can occur and be properly handled in subsequent processing operations, such as frequency shifting.
Advantageously, the signal processor is arranged to make a copy of the input signal and to frequency shift the copy by a frequency nominally equal to the separation between the first and second pilot tones.
Advantageously the pilot tones are identified by performing a frequency domain analysis of the sampled input signal. Various frequency domain analysis techniques may be used, but for speed and simplicity a fast Fourier transform may be performed. Advantageously the fast Fourier transform is restricted or windowed in order to compensate for the fact that it is operating on a data signal of limited temporal length. The fast Fourier transform is a complex fast Fourier transform in order to maintain both the magnitude and phase of each signal. The fast Fourier transform outputs the magnitude and phase of the input signal in a series of frequency bins (ie frequency slots). Advantageously, some of these bins may be blanked out in order to remove spurious components. The frequency domain results for the shifted and non shifted samples are then combined, taking account of their phase such that vectors due to the pilot signals have the same phase and tend to combine in magnitude. The signal processing is performed on signal buffers of fixed lengths. In order to improve the signal to noise ratio, the processed results of several buffers are averaged. Advantageously the spread of frequencies is correlated in order to combine energy in Doppler shifted versions of the pilot signals. Doppler shifting may occur because one of the radio units is moving with respect to the other, for example a base station may be mobile whereas the local transmitters connected to geophones are stationary, or because one or both the base station and remote units are moving with respect to the transmission media, as happens in marine seismic surveying whereby the hydrophones may be towed by a survey vessel.
The signal processor searches through the processed spectrum of the received signal to find the largest peak. Assuming that the input data was copied and the copy (or original) was up shifted in frequency, then the largest peak should correspond to the frequency of the upper pilot tone. Advantageously the signal processor is then arranged to estimate a measure of confidence in the fact that it has correctly identified the correct pilot tone. In order to do this, it may apply a two part test. As the first part of the test, the peak may be required to exceed a predetermined threshold either in absolute terms, or in respect to the amount by which it exceeds the remaining signals. Secondly, the magnitude of the largest peak may be compared to the magnitude of the second largest peak. This ratio may also be used to indicate the degree of confidence that a satisfactory lock has been achieved. If the difference between the magnitudes is small, then there is little confidence that the peak found is genuine. Thus the search process may be re-initiated.
Once the signal processor has found the pilot tones with sufficient confidence, a new control voltage for the oscillator is calculated and generated based upon the error between the expected pilot tone frequencies and the received pilot tone frequencies. If the frequency error is within predetermined limits, then the local oscillator is deemed to be locked, otherwise the local oscillator control voltage is updated in an attempt to establish an acceptable lock.
According to another aspect of the present invention, there is provided an apparatus arranged to frequency lock to a carrier signal comprising a pilot tone occurring in upper and lower sidebands of the carrier signal, characterised in that the apparatus is arranged to demodulate the received signal to recover a first pilot tone from the upper side band and a second pilot tone from the lower side band, and to locate the pilot tones by searching for signals separated by a frequency corresponding to an expected frequency separation between the first and second pilot tones, and generating a measure indicative of an error between the expected frequencies of the demodulated pilot tones and the actual frequencies of the demodulated pilot tones.
According to a further aspect of the present invention there is provided a method for reducing interference in a telemetry unit of a telemetry system, the method comprising generating all the clocks used in the telemetry units by locking them to a local master clock, such that any harmonic interference due to mixing of clock frequencies is predictable.
According to a further aspect of the present invention, there is provided a method of controlling clocks within a plurality of remote units in a distributed data collection system, comprising transmitting at least one reference signal from a master unit, and adjusting the clocks within the remote units to lock onto the reference signal.
Thus, once the local oscillator in each remote unit has been correctly locked by reducing the frequency error of the pilot tones to within an acceptable margin, it follows that the local oscillators of each remote unit are operating at substantially the same frequency.